An association fighting for the stolen children and their families, Anadir, presented the demand at the Madrid attorney general's office with evidence including DNA tests and testimony from nurses who admitted stealing babies.

The demand was made on behalf of the victims and families of 261 snatched babies. Anadir lawyer Enrique Vila said many others are expected to join the complaint.

"We get more and more calls from people who have doubts about their origins, because they have no physical resemblance with their parents or grandparents, or because their parents had them at an advanced age and they are single children," he said.

Anadir estimates there could be as many as 300,000 cases during the 1939-75 dictatorship and up to the end of the 1980s.

"It is very hard to know how many families are affected because many stolen children do not know they were stolen and will without a doubt die without ever finding out," said Mr Vila.

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Children of jailed left-wing opponents were stolen from their mothers with state approval and often the blessing of the Roman Catholic Church to purge Spain of Marxist influence.

A 1940 decree allowed the state to take children into custody if their "moral education" was at risk.

Historians say many of the "lost children" were put in Catholic religious orders and became nuns or priests while others were illegally adopted by other families – usually supporters of the regime – with changed identities.